The present invention relates to an image transfer device having a lens array laminate forming an erecting unit magnification optical system, the lens array laminate including a plurality of lens array sheets each of which has convex-convex lens elements arranged in a plurality of rows and which are closely laminated. This device is useful for a linear scanning optical system such as an optical printer or a scanner.
A gradient index rod lens array has been heretofore often used in an image transfer device for an optical writing system such as an optical printer or for an optical reading system such as a scanner. Great reduction in size of various kinds of apparatus such as an optical printer and a scanner can be achieved by the use of such a gradient index rod lens array. When the gradient index rod lens array is used, it may be however sometimes impossible to obtain sufficient resolution for an optical system requiring a high resolution of about 1200 dpi (dots per inch) in accordance with the design thereof.
Therefore, use of a planar lens array in place of the gradient index lens array has been proposed (e.g. see Japanese Patent Laid-Open No. 2000-221445). The optical system disclosed in this official gazette is constituted by 4n spherical lens array sheets (in which n is a positive integer and, accordingly, a 4-sheet configuration is the smallest with n equal to 1) Further, the distance between adjacent lens array sheets is set to be equal to the focal length of the lens (that is, a space is formed between adjacent lens array sheets) so that a high resolution of 1200 dpi can be achieved (but the focal length and the size of the lens are not described).
A member for keeping the distance between adjacent lens array sheets constant is however required for achieving an erecting unit magnification optical system in the configuration that the distance between adjacent lens array sheets is set to be equal to the focal length of the lens. Hence, the number of parts becomes large and assembling thereof becomes complex. Moreover, positioning accuracy becomes very stringent because both reduction in thickness of each lens array sheet and reduction in distance between adjacent lens array sheets are required for achieving high resolution.
Moreover, there is a problem that the number of lens array sheets used becomes large inevitably. In this example, at least four lens array sheets are required because the optical system is constituted by 4n lens array sheets.